The present invention relates generally to gas turbine engines, and, more specifically, to turbine rotor blades therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor and ignited for generating hot combustion gases. The gases are channeled to a turbine which extracts energy therefrom for powering the compressor and producing useful work, such as powering a fan for propelling an aircraft in flight.
A high pressure turbine first receives the combustion gases from the combustor and includes a stationary turbine nozzle followed in turn by a row of turbine rotor blades extending radially outwardly from a supporting disk. The nozzle includes airfoil vanes which direct the combustion gases to cooperating airfoils of the blades.
The vanes and blades are hollow and include various cooling circuits therein in which air is diverted from the compressor and used as a coolant for thermally protecting the vanes and the blades. The vane and blade cooling art is quite crowded due to the varying cooling requirements around the airfoils thereof from root to tip and between leading and trailing edges thereof.
The airfoils typically include several cooling circuits extending radially along the span of the airfoil and spaced axially for differently cooling the leading edge, mid-chord, and trailing edge regions of the airfoil.
The coolant channeled through the circuits removes heat by heat transfer convection inside the airfoils and is typically discharged through the pressure and suction sidewalls of the airfoil through film cooling holes which insulate the outer surface of the airfoil against the hot combustion gases which flow thereover.
Internal heat transfer may be increased by providing pins or straight ribs along the sidewalls of the airfoil which define turbulators that trip the coolant flow and locally increase heat transfer. Turbulators are found in various forms and orientations from perpendicular to the direction of coolant flow, as well as inclined relative thereto, as required for individual coolant channels of the different circuits.
Since turbine rotor blades rotate during operation, cooling thereof is rendered even more complex due to the rotary forces imposed upon the coolant. For example, Coriolis force acts upon the coolant flowing through the blades and affects the cooling ability thereof. In a typical radially extending cooling channel, the primary direction of the main coolant flow therethrough is either radially outwardly from root to tip of the blade or radially inwardly from tip to root of the blade, such as found in a typical multi-pass serpentine cooling circuit.
Typical straight turbulators may be orientated along the chords of the airfoil and generally perpendicular to the radial direction of the coolant flow, or may be inclined relative thereto, for correspondingly different performance. In both cases, however, the turbulators are effective for tripping the coolant locally along the inner surface of the airfoil for enhancing heat transfer thereat.
However, the Coriolis force affects cooling performance of the turbulators. The Coriolis force acts on the coolant in a direction perpendicular to the radial flow thereof according to the vector product of the radial velocity of the outwardly or inwardly directed coolant flow through the respective radial flow channels and the rotary speed of the blade about the axial centerline axis of the rotor disk. Accordingly, the Coriolis force acts on the coolant in opposite directions in an outward flow pass or channel as opposed to an inward flow pass or channel.
In both examples, however, the Coriolis force is effective for generating a pair of Coriolis vortices which counterrotate in each radial flow channel as a secondary flow field to the primary radially directed flow of the coolant. Each channel thusly develops a corresponding axially forward Coriolis vortex and an axially aft Coriolis vortex which rotate counter to each other, with different rotation in the inward and outward passes of the flow channels.
In U.S. Pat. No. 5,797,726-Lee turbulator pairs, also referred to as chevrons, are disclosed for cooperating with the Coriolis vortices for enhancing heat transfer cooling inside turbine rotor blades. The chevrons are directed differently along the pressure and suction sidewalls of the blade for cooperating with the Coriolis vortex pair in each cooling passage for locally directing coolant along the chevrons in the same direction as the adjoining Coriolis vortices as opposed to an opposite direction therewith. In this way, the tertiary local flow effect at the chevrons themselves is added to, instead of subtracted from, the secondary Coriolis vortices to prevent flow stagnation and thereby enhance heat transfer cooling inside the blade.
In view of the complexity of turbine airfoils and the directional affect of the Coriolis force, it is desired to further improve turbine blade turbulator design.